Make a copy of the script and give it a name (e.g. the name of the server it will upload to). The first time you drop a file on it the script will ask you for the destination URL. The easiest way to enter that is to connect to the right place with Fetch and choose Copy from the Edit menu, then Paste into the script's dialog.

From then on whenever you drop a file on the script it will direct Fetch to upload the file to the destination.